Ceramic honeycomb structures are commonly used in the art in the manufacture of filters for liquid and gaseous media, and in particular in the manufacture of filters for the removal of fine particles from exhaust gases; the filters are positioned in the exhaust lines of vehicle diesel engines, in order to remove the soot component of the exhausts. These filters can be monoliths or segmented ceramics honeycombs, which comprise cells or channels of dimension commonly ranging from 500 to 2000 microns, with controlled wall porosity. The cells are alternatively plugged on the inlet and outlet side, so that the exhaust gas is forced through the porous ceramic wall between the channels and filtration occurs when the gas crosses the wall.
Suitable honeycomb structures provide a balance of several desirable properties, such as sufficient filtering efficiency, i.e., the exhaust gas passing the filter should be substantially free of diesel particulates; limited pressure drop, i.e. the filter must show a sufficient ability to let the exhaust gas stream pass through its walls; and sufficient chemical resistance against the compounds present in exhaust gas of diesel engines over a broad temperature range.
A low thermal expansion coefficient and a high thermal shock resistance are also desirable, as they can help a filter to survive the several regeneration cycles that it normally undergoes during its lifetime, which involve rapid heating to temperatures substantially higher than the normal operating temperature. In fact, during filtering activities, the inlet channels of the honeycomb structures are progressively filled with soot, thus reducing filtering activities of the structures. Therefore the filter must be regenerated periodically; the cleaning of the filter is performed by heating the filter to a temperature sufficient to ignite the collected diesel particulates at high temperatures (normally higher than 1000° C.), thus causing the combustion of the soot. If filters do not possess sufficient thermal shock resistance, mechanical and/or thermal tensions may cause cracks in the ceramic material, resulting in a decrease or loss of filtering efficiency and consequently of the filter lifetime.
In order to increase the lifetime and their filtering efficiency of the honeycomb filters, various attempts have been made in the art to develop ceramic materials with improved properties, such as of silicon carbide (SIC), mullite, tialite or sillimanite minerals.
Further efforts have been directed to develop asymmetric designs of the cells, where the inlet cells are larger than the outlet cells; two main ways of creating asymmetry have been investigated in the art. The first solution comprises the use of curved walls of the channels, as described for instance in FIG. 6 of EP-A-1 676 622; in these designs the cells, which have normally square or rectangular cross-sections, may be partially deformed to create the asymmetry. As also shown in FIG. 1, all sides of the inlet cells are bulged outwardly (inlet channels are “inflated”) while the corresponding sides of the outlet cells are bulged inwardly to give a reduced cross-section area; the result is an undulation of the walls and a bulged pattern having the inlet cells of slightly greater area than the outlet cells. Nevertheless, this design requires the use of complex and costly dies in the manufacture of the filter; moreover, the many constraints accumulated in the structure may lead to problems with the ceramics performance. A further drawback of this solution is that adjacent inlet channels are very close to each other, thus decreasing filtration efficiency. Therefore, these designs have proved shortcomings when used for honeycomb filters, in particular for monolith filters.
A second way of creating asymmetry, known in the art, involves the use inlet channels having a cross-section higher than the cross-section of outlet channels, as shown in FIG. 2. For instance, WO 03/020407 describes a honeycomb structure wherein the cell channels have non-equal, square cross-section. This design has the disadvantage that the distance separating two adjacent inlet squares becomes smaller, thus creating areas of brittleness for the structure which may originate fractures. This drawback may be partly compensated by creating chamfers on the square, therefore creating octagonal cells; nevertheless, the surface of the chamfer leads to a decrease of filtering efficiency, as a significant portion of the inlet cell walls is closer to an adjacent inlet cell than to the nearest outlet cell, which necessitates a longer flow path through the wall.
Therefore, there is a need in the art for a new ceramic honeycomb structure having asymmetric design, able to provide honeycomb filters of increased lifetime and filtering efficiency, at the same time avoiding the problems of the asymmetric designs known in the art.